The invention relates to densifying hollow porous substrates by chemical vapor infiltration (CVI), and more particularly doing so at pressure equilibrium, i.e. without any pressure gradient between the faces of the substrates.
A particular field of application for the invention is making hollow parts out of a composite material comprising a porous substrate densified by a matrix, and in particular hollow parts made out of a thermostructural composite material.
The term “hollow porous substrate” is used herein to mean a substrate of hollow shape having a concave inside surface that is continuous, i.e. that does not have any holes, other than those due to the porous nature of the substrate. Examples of hollow shapes are spherical cap shapes, cylindrical or cylindro-conical shapes that are closed at one end, receptacle or bowl shapes, and cover shapes, not necessarily axially symmetrical. Examples of hollow parts obtained by densifying such substrates are receptacles for the chemical and metallurgical industries, such as crucibles or crucible-support bowls, or protective bodies for spacecraft, such as nosecones that form heat shields.
The term “thermostructural composite material” is used herein to mean a composite material which presents both good mechanical properties enabling it to constitute structural elements, and the ability to conserve these properties at high temperatures. Examples of thermostructural composite materials are carbon/carbon (C/C) composite materials comprising a porous substrate of carbon fibers densified by a carbon matrix, and ceramic matrix composite (CMC) materials comprising a refractory porous substrate, e.g. of carbon or ceramic fibers, densified by a ceramic matrix.
Densifying porous substrates by chemical vapor infiltration at pressure equilibrium is a well-known process. The substrates to be densified are placed in an enclosure and a reactive gas is admitted into the enclosure where the temperature and pressure conditions are controlled in such a manner that the gas diffuses into the pores of the substrates and forms therein a solid deposit that constitutes the matrix of the composite material either by causing a precursor gas to decompose or by causing a reaction to take place between a plurality of precursor gases contained in the gas. This applies, for example, to a carbon matrix where the gaseous precursor is generally an alkane, an alkyl, or an alkene, such as propane or methane or a mixture thereof. For a ceramic matrix made of silicon carbide, for example, the gaseous precursor is methyltrichlorosilane.
In general, the reactive gas is admitted into one end of the enclosure, while the effluent gas comprising the residue of the admitted gas and any reaction products is extracted from the other end. Admission advantageously takes place through a preheater zone serving to bring the admitted gas up to a temperature close to the temperature of the substrates for densifying.
A plurality of substrates can be densified simultaneously in a single enclosure, with the substrates being placed in such a manner as to ensure that all of them are exposed to the flow of reactive gas admitted into the enclosure. U.S. Pat. Ser. No. 5,904,957 shows identical substrates disposed in a particular manner in annular stacks for this purpose. The gas is admitted and directed towards the inside (or the outside) of the stacks and it flows through the substrates and the gaps between them so as to be taken up from the outside (or the inside) of the stacks.
When the substrates to be densified are hollow as defined above, having relatively deep concave portions, and in particular when they are of quite large dimensions, defects have been observed by the inventors after densification by chemical vapor infiltration. These defects consist in variations in the microstructure of the material of the matrix between different portions of the densified parts, and in the formation of soot or undesirable projections on the substrates.
Since these defects can be attributed to excessive maturation of the admitted gas, i.e. the admitted gas spends too long in transit inside the enclosure and therefore ages, thereby spoiling its properties, attempts have been made to remedy that problem by increasing the gas flow rate by increasing the rate at which effluent gas is pumped out from the enclosure. However that has not made it possible to eliminate the defects completely, while it has considerably increased cost by increasing consumption of reactive gas.